Adblock Plus — Blocks or hides advertisments and other annoyances from web pages, while still retaining the correct page format. It is designed to allow you to block flash and image banners built into the web page, as well as scripts. You add your own filters using wildcards (*) for simple filters, or RegEx (regular expresions) for advanced ones. An excellent maintained set of filters is here.

Flashblock — Allows you to easily block flash animations. Flash animations are replaced with an icon you click to play. May conflict with Adblock Plus and crash on some flash-intensive sites.

Image Zoom — Adds zoom functionality for images. Functionality is also included in the Mouse Gestures extension.

Focus Last Selected Tab — Brings focus to the last selected tab when closing the active tab. Functionality is also included in the TabMix extension.

Session Saver — Saves the open tabs when closing the browser, and restores them upon restarting it. Functionality is also included in the TabMix extension.

Single Window — A simple extension that allows Mozilla to fully utilize the built-in tabbed browsing behavior. Traps links that would normally open in a new window. Functionality is also included in the TabMix extension.

Tabbrowser Preferences — Enables enhanced control for tabbed browsing. Most functionality is also included in the TabMix extension. Recommended for users who would like to change the way tabs behave in Firefox.

Tab Mix — An extension with lots of features, including opening closed tabs, moving tab bar to the bottom, mouse gestures and much more. It is incompatible with various extensions that offer similar funcionality.

Undoclosetab — Reopen a closed tab. Functionality is also included in the TabMix, SessionSaver, and Mouse Gestures extensions.